The present invention relates to an apparatus for treating pulp. The object of the present invention is especially to develop the disc filter used in the pulp and paper industry to meet the requirements of the present time.
A disc filter comprises, as known, wire-coated discs mounted on a horizontal shaft in a vessel containing e.g. fiber suspension. The discs comprise a number of wire-coated sectors, in the inside of which a suction is generated one way or the other for removing liquid inside the sector from a pulp layer accumulating on the wire surface. The filtrate thus obtained is supplied to a suction apparatus through a hollow shaft of the filter or from each sector through a filtrate channel leading to a valve apparatus at the end of the shaft. Said suction apparatus may be a suction leg or even a centrifugal pump. It is a typical feature of the disc filters that the ratio of the filtering surface of the filter to the outer dimensions thereof is very large compared to a drum filter, which is another alternative in the industry. On the other hand, now that drum filters operating on a multi-stage principle have been developed, it must be noted that disc filters according to the prior art are typically one-stage apparatuses.
Our invention is partially based on a disc filter disclosed in U.S. Pat. No. 4,123,363. Said disc filter represents one of the most developed disc filter types, because the hollow shaft interior is provided with a stationary slide/valve in such a way that said space is divided into axial chambers, of which each may be separately connected to a suction apparatus of its own. This means in practice that a number of different filtrates may be removed from the disc filter. In most cases this feature is utilized in such a way that the so called initial filtrate obtained at the beginning of each filtering stage and the actual, clear filtrate are separated. However, a disc filter in accordance with this patent is still a single-stage apparatus, from which both filtrates are removed separately or the so called initial filtrate is possibly supplied back to the feed of the suspension entering in the filter.
In order for a disc filter to maintain a competent position relative to a drum filter, a multistage version of the disc filter should be developed, whereby it would become possible to replace the previously regularly used two disc filters with a single two-stage filter. The savings thus gained both in space and in equipment investments would be significant.
The purpose of the present invention is to eliminate said restriction of the disc filter and to make such improvements in the disc filter that it may be used, if so desired, as a multistage apparatus.
A characterizing feature of a disc filter in accordance with the present invention is that the tank and each of the flow channels in the shaft are divided into at least two chambers separated from each other by radially extending partitions.